fix: correct the trigger function to make sense for the given example (supabase#41079)

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Co-authored-by: Chris Chinchilla <[email protected]>

Author: filipecabaco

apps/docs/content/guides/realtime/subscribing-to-database-changes.mdx

@@ -41,7 +41,7 @@ language plpgsql
 as $$
 begin
   perform realtime.broadcast_changes(
-    'topic:' || coalesce(NEW.topic, OLD.topic) ::text, -- topic - the topic to which we're broadcasting
+    'topic:' || coalesce(NEW.id, OLD.id) ::text,       -- topic - the topic to which you're broadcasting where you can use the topic id to build the topic name
     TG_OP,                                             -- event - the event that triggered the function
     TG_OP,                                             -- operation - the operation that triggered the function
     TG_TABLE_NAME,                                     -- table - the table that caused the trigger